1. Field of the Invention
This invention relates to electronic circuitry, and more particularly to feedback amplifier output stages with an output voltage that swings between high and low levels as determined by a variable input signal.
2. Description of the Prior Art
It is desirable that output stages for feedback amplifiers, such as operational amplifiers (op amps), have a wide output voltage swing over the range of input signals. A conventional output stage is shown in FIG. 1. As described below, it is characterized by a somewhat limited output voltage swing; the present invention seeks to improve upon this characteristic.
The output stage has a pair of bipolar transistors Q1 and Q2 that are connected in series between high and low voltage buses V+ and V-. By "high" and "low" it is meant that V+ is at a higher relative potential than V-, not necessarily that the absolute value of V+ is greater than that of V- (although V- may typically be at ground potential). Q1 is an npn device while Q2 is pnp so that while one transistor is ON the other is OFF, as explained in detail below.
A current source I1 is connected on one side to V+ and on the other side to drive current into the base of Q1 and to a series-connected diode circuit consisting of diodes D1 and D2. The diodes are oriented to conduct current from I1 to the base of Q2.
An input bipolar transistor Q3 has its base connected to the output of the feedback amplifier, such as op amp 2, for which the circuit serves as an output stage. The collector-emitter circuit of Q3 is connected between the diode circuit and V-, with the collector of Q3 connected in common with the base of output transistor Q2. An output terminal T1 from the circuit is taken between the emitters of Q1 and Q2.
Ideally, the voltage at T1 should equal V+ when the input signal to Q3 from op amp 2 is in one state, and should equal V- when the input signal is in its opposite state. Another requirement of the circuit is for rapid switching; D1 and D2 supply quiescent current to the output transistors for this purpose. Without the quiescent transistor current, a time delay would have to be incurred each time a transistor was turned on to allow enough time for charging the transistor. Also, the provision of quiescent transistor current inhibits high crossover distortion. Two diodes are required because there are two base-emitter circuits between the bases of Q1 and Q2.
To explain the operation of the output stage, assume that the input signal from op amp 2 is initially low, turning Q3 OFF. This causes the current from I1 to flow into the bases of both Q1 and Q2, turning npn device Q1 ON and pnp device Q2 OFF. The output voltage at T1 thus goes to V+.
Assume now that the input signal from op amp 2 switches to high. This causes Q3 to switch to a conductive state, shorting the base of Q2 to V- to turn Q2 ON. The current from I1 is diverted away from the base of Q1 and instead flows to V- through Q3, causing Q1 to turn OFF. The desired result is to set the output voltage at T1 to V-. However, the presence of diodes D1 and D2 prevents the output voltage from swinging all the way to V-. This is because diodes D1 and D2 are implemented in a conventional manner as npn transistors with their bases and collectors connected together as the diode anode, and their emitters functioning as the diode cathodes. Thus, there is a base-emitter voltage drop (approximately 0.7 volts) across each diode. The voltage at the base of Q1 is therefore two base-emitter drops above V-. This voltage drops by one base-emitter increment from the base of Q1 to T1, leaving the voltage at T1 still one base-emitter drop above V-.